This invention relates to welding methods and to a novel backup for use therein; the invention is particularly concerned with single pass, high temperature, high current, or heavy plate welding.
In the manufacture of large tanks, ships, etc., metal plates are often joined together by abutting their edges and joining them with a weld seam. It is highly desirable, if not absolutely essential, that there be a complete penetration from face to root (i.e., from the welding side to the opposite side) of the welded material. To insure that a 100% weld seam is formed (i.e., that the weld material penetrates completely from the face to the root) one conventional practice has been to form the weld seam by welding in multiple passes on each side of the plates to be joined. This procedure produces sound welds, but is expensive and time consuming because of the need to weld from two sides.
Various methods have been proposed which permit metal plates to be welded from one side. For example, rectangular, square or bar-like welding backups have been employed. A welding backup is centrally positioned behind the plates to be welded so as to contain the molten material generated in the welding process. As the molten material cools it solidifies into a weld seam or "bead" that requires no additional welding from the opposite side of the plates. Metallic backup bars made from copper or a metal similar to the plate to be welded have been used in combination with an inorganic parting layer. This combination prevents the joining of the backup to the weld.
Non-metallic welding backups are particularly advantageous, as described in R. H. Keith "Weld Backings Come of Age" June 1975, Welding Journal, p. 422; since they do not involve many of the cost and implementation disadvantages of metallic weld backings. Heat resistant tapes, e.g., as described in U.S. Pat. No. 3,001,057 (Hackman et al); and U.S. Pat. No. 3,138,863 (Clark et al) and rigid heat resistant ceramic tile or glass materials, e.g., as described in U.S. Pat. No. 2,820,427 (Chyle et al) are exemplary non-metallic welding backup materials. The present invention relates generally to non-metallic (e.g., ceramic) welding backups.
Conventional ceramic welding backups generally have working surfaces that are either flat or that have a single longitudinal groove therein. Referring to FIG. 1, two relatively heavy (e.g. 12 mm or over) parent metal plates 10 have been positioned in slightly spaced apart relationship so that they may be welded from above. A prior art ceramic welding backup 12 having an essentially flat working surface 14 is positioned beneath plates 10 so as to contain a puddle of molten weld material 11 (i.e., parent metal, filler metal and welding flux) generated in the "V" defined by the angle cut plate edges 16. The heat generated in welding melts the weld material and in turn the ceramic tile 12 (the melted portions of the plates 10 and backup 12 being shown in phantom) thus creating in the tile a central zone of melted ceramic. As the ceramic material is melted, the more dense weld material displaces the melted ceramic creating on the tile working surface a concave channel 18. The displaced hot ceramic material, in turn, displaces molten parent metal producing indentations or undercuts 20 in the parent metal plate. As the molten metal and ceramic material cool, highly undesirable undercut welds corresponding to indentations 20 remain when the ceramic backup is removed.
In an attempt to eliminate the formation of undercut welds which tend to diminish the strength of a weld seam, ceramic backup tiles having single longitudinal grooves have previously been employed. As depicted in FIG. 2, a ceramic backup tile 22 having on its working face 24 a single groove 26 (shown in phantom) is positioned beneath the spaced-apart plates 10 so as to cooperate therewith to contain a puddle of molten weld material 11. As described above, the molten weld material melts the ceramic backup. However, little or no ridges or undercuts are formed because the molten weld materials and molten ceramic flow into the groove 26 and thus are not deleteriously forced against the plates 10. Unfortunately conventional back-up tiles that are grooved deeply enough and wide enough to prevent the formation of undercut welds produce a large and equally objectionable backside reinforcement 28. Backside reinforcements which are too large must be machined off, requiring additional labor and expense.
The present invention provides grooved, preferably ceramic, welding backups that reduce the tendency to form undercut welds, and produce a relatively small backside reinforcement. Moreover, the present invention provides welding backups which permit the maximum thickness of the backup to be positioned adjacent the hottest sector of the weld, thereby minimizing backup burn-through (i.e., burning through the backup by the molten weld material).